In typical large-scale industrial mixing and pumping applications, a radial flow impeller, also referred to as a "pumper impeller," is disposed near the bottom of a tank filled with the liquid media to be mixed and to be pumped or just to be pumped through an outlet port of the tank located in an upper portion thereof. Such impellers, frequently open-faced on one face thereof and at least partially open-faced on another face thereof, are rotatably driven by a drive shaft which extends from the impeller to a gear box drive means usually positioned above the tank. Impeller rotation imparts to the liquid or liquids forces which generate in the liquid medium a so-called "head," a measure of the pressure the pumper impeller would generate in the liquid if the tank were completely closed. When the tank has an outlet port, the "head" provides flow of liquid through the outlet, the flow is principally commensurate with the pumping effectiveness of the impeller, the tank configuration and the volume and viscosity of the liquid or liquids to be pumped. The efficiency of pumping may be expressed as the product of the head and flow and a constant divided by the power applied to drive the impeller.
Various industrial mixing and pumping processes are based upon a "flow-through" principle, wherein a liquid or liquids are continuously provided at an inlet port for such liquids, frequently located at or integrated with the tank bottom. The radial flow impeller is usually arranged concentrically with the inlet port and in proximity thereto.
In the aforementioned applications of an open-faced "pumper impeller," the "head" and flow can be increased, at least in principle, by increasing the impeller's rotational speed. Such speed increase requires a higher power input to the drive shaft (and to the gear box drive means), and may result in accelerated wear and/or reduced mechanical integrity of the impeller, the drive shaft and the gear box drive means.
In addition to the aspects of "head" and flow, certain industrial tank-based mixing and pumping processes call for particular outcomes of a mixing and pumping process. For example, so-called solvent extraction processes have a first stage, referred to as mixer tank, in which a dispersion of droplets of one liquid is to be formed in another immiscible liquid by the action of an impeller, and the dispersion is to be pumped through an outlet port to subsequent process stages.
Briefly described, in a solvent extraction process one liquid is an aqueous liquid comprising a solution of metals in dilute sulfuric acid (derived in a prior leaching operation), and another liquid comprises organic fluids (for example kerosene and an extractant). These liquids are provided to the mixer tank through an inlet port (also referred to as an "orifice") located in the bottom of the mixer tank. Generally, a single radial flow inducing impeller (a "pumper impeller") is used near the bottom of the tank to pump the liquids, thereby mixing them and creating a dispersion of droplets of either the organic liquid or liquids in the aqueous liquids or, alternatively, to form a droplet dispersion of the aqueous liquids in the organic liquids, the organic liquid being referred to as the solvent. The selection of the one liquid which will form a droplet dispersion in the other, immiscible liquid, depends on numerous factors, including considerations of the respective liquid volumes, flow rates, choice of aqueous and organic liquids, as well as design considerations pertaining to the mixer tank and the impeller. The mixer tank has a baffled overflow region or weir through which the liquid droplet dispersion enters into a number of successive stirring tanks, eventually to reach a so-called settler stage in which the aqueous phase and the organic phase (the solvent) settle out by coalescence of the dispersed droplets. At respective outputs of the settler stage, the organic and aqueous liquids are drawn off for further processing steps in which the metal to be produced is extracted from the organic or the aqueous liquids (depending on whether the droplet dispersion was formed as solvent droplets in the aqueous continuous phase or as aqueous droplets dispersed in the organic continuous phase), and the solvent liquids are recovered for eventual recycling into the mixer tank. Since a large-scale industrial metallurgical solvent extraction process requires a substantial and continuous quantity of relatively costly organic (solvent) liquids, economic considerations drive the effectiveness of solvent extraction and solvent recovery.
For this reason, a central issue in such flow-through solvent extraction processes is the droplet size distribution of the droplet dispersion formed in the mixer tank under selected input flow rates of liquids for a selected impeller, tank design, and power level applied to the impeller shaft at a certain impeller rotational speed. A second issue is the efficiency of droplet formation while pumping and mixing the dispersion, also referred to as hydraulic efficiency, under certain operating conditions of the mixer tank.
With respect to the size distribution of the droplet dispersion, it is well known that the mass transfer coefficient (a measure of the ability of transferring a mass of one liquid in a dispersed state into another liquid) increases significantly as the droplet size decreases. On the other hand, the coalescence rate of droplets in the dispersion increases rapidly with increasing droplet size, particularly at larger droplet diameters, thus potentially resulting in premature coalescence of droplets into a continuous phase prior to the dispersion reaching the settler stage of the solvent extraction system.
When the droplet size distribution of the dispersion generated in the mixer tank is shifted toward small droplet diameters, such as microdroplets (also called "fines"), a phenomenon referred to as entrainment may adversely affect the downstream refining process of the metal, since, for example, fines of the organic liquids (solvent) may be permanently entrained in the aqueous phase at the settler stage of the process. Such entrainment also reduces the effectiveness of solvent recovery, since permanently entrained solvent droplets effectively constitute a loss of the organic liquids (solvent) in the case of the above example. Therefore, in order to resolve the potentially conflicting requirement of a desirably high mass transfer coefficient at small droplet sizes of the dispersion, having the attendant potential difficulty of entrainment, and the potentially premature coalescence of larger sized droplets, it is desirable to form in the mixer tank a relatively narrow droplet size distribution of the dispersion, an optimum droplet size approximately centered on a droplet diameter at which an acceptable mass transfer coefficient is desirably achieved with minimum potential for entrainment and yet having an acceptable droplet coalescence rate.
Even if operating conditions of a mixer tank do not yield such an ideal relatively narrow droplet size distribution, it is desirable to form a dispersion of non-entraining droplets or, stated differently, it is desirable to form a droplet dispersion devoid of very small droplets (microdroplets) prone to entrainment.
Some of the aforementioned considerations on the performance of a mixer tank of a solvent extraction plant, as well as other aspects thereof, have been described by Warwick and Scuffham in a publication entitled The design of mixer-settlers for metallurgical duties in the journal, Hel Ingenieursblad, 41e jaargang (1972), nr. 15.16, pages 442-449, and by Lott, Warwick, and Scuffham in a paper entitled The design of large scale mixer settlers, presented at the AIME Centennial Annual Meeting in 1971.
These authors describe the design of mixer-settlers of a solvent extraction process using a single pump-mix impeller with curved blades in the mixer tank to generate the dispersion of droplets from the organic and aqueous liquids, the mixer tank being followed immediately by a settler stage. Since the early 1970's, solvent extraction plants have evolved which include in their design one or several stirrer tanks disposed between the mixer tank and the settler stage.
As indicated in the foregoing, in a tank-based pumping system it is desirable to pump a liquid or liquids efficiently and with an enhanced flow through an outlet port of the tank by an impeller in the tank. Such enhanced pumping at a given power applied to the impeller, and alternatively efficient but non-enhanced pumping at a reduced impeller power input level, is desirable in applications using a liquid-filled tank with a closed tank bottom and in so-called flow-through systems.
In tank-based, flow-through pumping and/or mixing systems, it is desirable to pump and/or mix liquids with an enhanced liquid flow. In a particular, pumping and mixing process designed for effective operation of a mixer tank of a metallurgical solvent extraction facility, it is desirable to achieve enhanced-flow pumping and mixing of at least two immiscible liquids so as to form a dispersion of droplets of at least one liquid in at least one other liquid, wherein droplet sizes are desirably produced which result in non-entraining conditions in subsequent process stages of such a facility.